In the field of wrapping or packaging materials for foods and industrial materials and of automotive materials, single-substance materials have been unable to find their uses, and efforts have been exerted to develop composite materials, polymer alloys, and the like applicable to such fields. Although laminates are among these materials, the laminates produced so far do not necessarily have desired properties.
One of the major reasons for the above is that the adhesion between laminae may be insufficient because the respective laminae to be united into a laminate often have contradictory natures. In order to eliminate this problem, various kinds of adhesive resins have been proposed so far, but it has been difficult to always meet property requirements that are being diversified and becoming severe.
Polyolefins are used in various applications owing to their low cost, good mechanical strength, hygienic nature, and good formability. However, since they are poor in oil resistance and gas barrier properties, polyolefin packages for foods such as mayonnaise and soy sauce cannot preserve such foods over prolonged periods of time. In addition, polyolefin containers for gasoline are defective in that a large proportion of the gasoline in the containers permeates through the container walls and is lost, and that the containers are swollen by the gasoline to suffer deformation.
Although various proposals have been made so far for the purpose of eliminating these defects of polyolefins, the results are complicated production processes, increased costs, and limitations in use and design, so that satisfactory results have not been obtained.
Among such proposals are lamination of polyolefins to materials that compensate the defects of polyolefins, such as polyesters, polyamides, saponified ethylene-vinyl acetate copolymers, aluminum foil, glass, etc., and vapor deposition of metals or inorganic substances on polyolefins. However, polyolefins by nature have poor affinities for the above resins and inorganic substances due to their chemical structures and, hence, lamination of polyolefins to such substances is difficult. Although it has been proposed to provide an adhesive layer between laminae to solve the above problem, this newly necessitates a step of applying an adhesive, disadvantageously resulting in a complicated production process.
Well known as adhesive resins for use as laminae to be coated on supports are polyolefins which have been modified by grafting thereonto unsaturated carboxylic acids or anhydrides. However, laminates containing laminae of such modified polyolefins have still insufficient adhesion, so that lamina separation often occurs. Such laminates are also defective in that since the production process involves a modification step in addition to polymerization, production costs are raised.
Laminates containing polyolefin resins have conventionally been used as various kinds of wrapping or packaging materials because they are excellent in the properties required of wrapping or packaging materials, such as cost, handling properties, productivity, and heat-sealing properties. However, since polyolefin resins are nonpolar in themselves, laminating thereof to other kinds of materials has necessitated modification of the polyolefin resins by graft polymerization, flame treatment or corona discharge treatment of substrates, application of anchor coats on substrates, or other measures.
In the case where laminates containing polyolefin resin laminae are to be obtained by extrusion laminating, an extrusion-coating technique is often utilized which, for example, comprises applying a kind of adhesive called an anchor coat and then extrusion-coating a polyolefin resin. Use of anchor coats, however, is defective in that the solvents should be recovered to prevent pollution of the working atmosphere and that production efficiency, cost, safety, etc. are adversely affected. In addition, it is difficult to impart sufficient adhesion properties only by the application of an anchor coat, so that it has often been necessary to form polyolefin resins into sheet at exceedingly high temperatures (300.degree. C. or higher) to oxidize the resin surface before the resins are processed into laminates.
Because of such production processes, the polyolefin resins in the final laminates have deteriorated to emit strong odors, so that the laminates cause problems when used as wrapping or packaging materials for foods.
In recent years, in particular, wrapping and packaging materials are required to meet requirements that are becoming more and more severe so as to eliminate the adverse influence of the materials on the smells, flavors, etc. of foods. Under such circumstances, an increasing number of laminates produced by conventional processes are being considered to be unusable.
Although the adverse influence on smells and flavors can be diminished to some degree by performing the processing of polyolefins at lower temperatures (280.degree. C. or lower), this results in difficulties in producing good laminates because adhesion to the substrates is insufficient.
Another method for ensuring adhesion to substrates is to utilize copolymers of ethylene and polar comonomers as the polyolefin resin. This method is being extensively employed.
As such copolymers, use is made of ethylene-ethyl acrylate copolymers, ethylene-acrylic acid copolymers, ethylene-methacrylic acid copolymers, ethylene-ethyl acrylate-maleic anhydride copolymers, and the like. However, these copolymers have strong odors attributable to the comonomers although they show improved low-temperature adhesion, so that it is difficult to produce, even at low temperatures, laminates usable for food wrapping or packaging. The applications of such laminates may be widened to some extent by washing those copolymers with solvents, etc. to remove substances that cause the odors.
However, the above method is not efficient because the production of such copolymers is quite costly, since large-scale equipments are necessary to washing with solvents and recovery of the solvents and care should be taken because of the toxicity and flammability of the solvents.
The dry lamination method, among laminate-producing processes besides the extrusion laminating, can yield laminates with good properties. The dry lamination process, however, is inferior to the extrusion laminating in working efficiency and production cost, and is hence not an efficient process.